1. Field of the Invention
The present invention relates generally to a hydraulic control apparatus for a hydraulically operated automatic transmission of an automotive vehicle, and more particularly to improvements in such a hydraulic control apparatus, for minimizing shifting shocks of the transmission.
2. Discussion of the Prior Art
An automatic transmission for a motor vehicle incorporates a plurality of hydraulically operated frictional coupling devices such as clutches and brakes, which are controlled by a hydraulic control apparatus, which includes shift valves for shifting the frictional coupling device to selectively establish appropriate shift positions of the transmission. For regulating hydraulic pressures applied to the frictional coupling devices during their engaging actions, the hydraulic control apparatus employs suitable accumulators of a cylinder-and-piston type.
Such an accumulator is adapted to maintain a predetermined optimum hydraulic pressure to be applied to the frictional coupling device for a suitable length of time, so as to reduce the shifting shock of the transmission.
The optimum hydraulic pressure level to be established by the accumulator varies depending upon the torque of the vehicle engine transmitted to the automatic transmission. This optimum pressure level may be controlled by regulating the back pressure to be applied to a back pressure chamber of the accumulator.
In the light of the above, a manner of precisely regulating the back pressure of the accumulator is proposed in laid-open publication No. 61-130653 (published in 1986) of unexamined Japanese Patent Application, wherein the duty cycle (ratio of the ON time to the OFF time) of a solenoid-operated accumulator control valve unit for regulating the back pressure of the accumulator is controlled depending upon the currently detected opening of a throttle valve of the vehicle engine.
Described more specifically, the hydraulic control apparatus uses a predetermined relationship between the optimum duty cycle of a solenoid valve of the accumulator control valve unit and the throttle opening, and controls the duty cycle of the solenoid valve according to this predetermined relationship. Accordingly, the duty cycle is fixed for a specific angle of opening of the throttle valve, whereby the back pressure of the accumulator is fixed for that specific throttle opening. Accordingly, the hydraulic pressure which is established by the accumulator is fixed when the frictional coupling device is shifted with the same throttle opening. Refer to FIGS. 7(A)-7(D).
In the actual operating conditions of the automatic transmission, the engine torque varies to a relatively large extent, depending upon the engine speed, temperature and pressure (supercharger pressure) of the intake air into the engine, and other factors, even when the throttle opening is held constant. Accordingly, the established back pressure of the accumulator tends to fluctuate off the optimum level.
A solution to the above drawback is proposed for example in laid-open publication No. 61-149657 (published in 1986) of unexamined Japanese Patent Application. The proposed solution is adapted to intricately regulate the back pressure of the accumulator, based on not only the throttle opening, but also various other operating parameters or factors such as the temperature of the working fluid in the automatic transmission, temperature of the intake air into the engine, engine speed, and supercharger pressure.
However, there inherently exist variations or fluctuations of the operating parameters that cannot be expected or predicted at the time of designing the hydraulic control apparatus. Further, chronological changes of the operating characteristics of the transmission and engine are inevitably encountered. Therefore, it is difficult to accurately control the hydraulic pressure to be applied to the frictional coupling device, even if the various vehicle running conditions or transmission and engine operating parameters are taken into consideration in controlling the duty cycle of the solenoid valve of the accumulator control valve unit.
For instance, the output torque of the engine inevitably varies with the chronological change of the engine per se, causing operating characteristics of the automatic transmission to be changed, even if the throttle opening, engine speed, and temperature and pressure of the engine intake air are held constant.
Furthermore, the dimensional accuracy and operating characteristics of the valves and accumulators for one transmission differ from those for another transmission of the same construction. This difference results in different optimum levels of the hydraulic pressure applied to the individual transmission units. Even if the transmission units are tuned for consistent operating characteristics at the time of manufacture, the characteristics unavoidably vary during use of the transmission units.
It is also noted that the working fluid in the automatic transmission deteriorates and may contain foreign matters, giving an adverse effect on the flows of the fluid and consequently on the operation of the transmission.
Also, the dynamic friction coefficient of the frictional coupling device of the transmission is lowered due to wear of the coupling device during its use. A graph in FIG. 8 shows a relationship between the cumulative running distance or mileage of the vehicle and the dynamic friction coefficient .mu.d of the frictional coupling device. A reduction in the dynamic friction efficient .mu.d causes an appreciable change in the operating or shifting characteristics of the friction coupling device.
In the presence of the unexpectable or unforeseeable factors indicated above, it is difficult to obtain a satisfactory degree of consistency in the operating characteristics of the automatic transmission, according to the known hydraulic control arrangement wherein the accumulator back pressure is regulated according to a predetermined relationship between the duty cycle of the accumulator control valve unit and a certain operating parameter or parameters such as the throttle opening.
Referring to FIG. 9, the disadvantageous aspect of the known hydraulic control arrangement will be further clarified. Graphs in FIG. 9 show a change in the output shaft torque To of the engine, during a typical shift-up operation of an automatic transmission which occurs when the accelerator pedal of the vehicle is depressed by a considerably large amount. A solid-line curve indicates the torque To while the engine and automatic transmission of the vehicle are operated in a normal condition. A dashed-line curve indicates the torque To when the engine torque level is generally lowered. Further, a curve in one-dot chain line indicates the torque To when the dynamic friction coefficient .mu.d of the frictional coupling device of the transmission is lowered.
In the case where the accumulator back pressure is regulated based only on the throttle opening, the hydraulic pressure level established by the accumulator is not changed even if the engine torque is lowered, provided the throttle opening is constant. In this case, a shifting operation of the coupling device occurs at a relatively high rate, and the shifting operation is terminated in a relatiely short time, as indicated by the dashed-line curve of FIG. 9.
In the case where the accumulator back pressure is regulated based on the engine torque itself, the back pressure may be changed with the engine torque However, a reduction in the dynamic friction coefficient .mu.d of the frictional coupling device results in a decrease in the torque transmitting capacity of the friction members involved, leading to an increased shifting time of the coupling device, even if the accumulator back pressure is kept constant. In an extreme case, the shifting action of the coupling device is not completed even when the piston of the accumulator reaches its operating end. In this instance, the coupling device is subject to an extremely large shock.
The above-indicated shock arises from a sudden increase in the torque transmitting capacity of the frictional coupling device, immediately after the operating end of the accumulator piston is reached. However, it is very difficult to design the hydraulic control apparatus so as to successfully deal with the reduction in the dynamic friction coefficient .mu.d of the frictional coupling device, as described above.